The Rebel Worlds / Knight of Ghosts & Shadows

The Rebel Worlds (1969)
The barbarians in their long ships waiting at the edge of the Galaxy waited for the ancient Terran Empire to fall, while two struggled to save it: ex-Admiral McCormac, forced to rebel against a corrupt Emperor, and Starship Commander Flandry, the brilliant young officer who served the Imperium even as he scorned it. Trapped between them was the woman they both loved, but couldn't share: the beautiful Kathryn - whose single word could decide the fate of a billion suns.

A Knight of Ghosts and Shadows (1975)
Raconteur, bon vivant, troubleshooter for the decaying Terran Empire, Dominic Falndry doesn't crave further danger in the service of galactic unity. But duty calls, so it's back to the spaceways for the most elegant Special Agent is a hundred star systems--straight into the well-laid plans of his lifelong enemy Aycharaych. Win or lose, though, the long night of human civilization is coming and Flandry knows it. How many more battles can he stand to win in a losing cause? And how many planets will die meanwhile?